Conventionally, in a thin film magnetic recording medium, a lubricant is applied onto a surface of a magnetic layer for the purpose of reducing frictions between a magnetic head and the surface of the magnetic recording medium, or reducing abrasion. In order to avoid adhesion, such as sticktion, an actual film thickness of the lubricant is of a molecular order. Accordingly, it is not exaggeration to say that the most important thing for a thin film magnetic recording medium is to select a lubricant having excellent abrasion resistance in any environment.
During a life of a magnetic recording medium, it is important that a lubricant is present on a surface of the medium without causing desorption, spin-off, and chemical deteriorations. Making the lubricant present on a surface of a medium is more difficult, as the surface of the thin film magnetic recording medium is smoother. This is because the thin film magnetic recording medium does not have an ability of replenishing a lubricant as with a coating-type magnetic recording medium.
In the case where an adhesion force between a lubricant and a protective film disposed at a surface of a magnetic layer is weak, moreover, a film thickness of the lubricant is reduced during heating or sliding hence accelerating abrasion. Therefore, a large amount of the lubricant is required. The large amount of the lubricant is the mobile lubricant, and therefore a function of replenishing the lost lubricant can be provided. However, an excessive amount of the lubricant makes the film thickness of the lubricant larger than the surface roughness. Therefore, a problem associated with adhesion arises, and in a crucial case, sticktion arises to cause driving failures. These problems associated with frictions have not been sufficiently solved by conventional perfluoropolyether (PFPE)-based lubricants.
Particularly for a thin film magnetic recording medium having high surface smoothness, a novel lubricant is designed at a molecular level, and synthesized to solve the above-described trade-off. Moreover, there are a number of reports regarding lubricity of PFPE. As described, lubricants are very important in magnetic recording media.
Chemical structures of typical PFPE-based lubricants are depicted in Table 1.
TABLE 1Fomblin-based lubricantsX—CF2(OCF2CF2)n(OCF2)mOCF2—X(0.5 < n/m < 1)ZX = —OCF3Z-DOLX = —CH2OHZ-DIACX = —COOHZ-TetraolX = —CH2OCH2CHCH2OHOH AM2001 Other lubricantsA20H MonoF—(CF2CF2CF2O)1—CF2CF2CH2—N(C3H7)2
Z-DOL in Table 1 is one of lubricants typically used for thin-film magnetic recording media. Moreover, Z-Tetraol (ZTMD) is a lubricant, in which a functional hydroxyl group is further introduced into a main chain of PFPE, and it has been reported that use of Z-Tetraol enhances reliability of a drive while reducing a space at an interface between a head and a medium. It has been reported that A20H suppresses decomposition of the PFPE main chain with Lewis acid or Lewis base, and improves tribological properties. On the other hand, it has been reported that Mono has a different polymer main chain and different polar groups to those of the PFPE, and the polymer main chain and polar groups of Mono are respectively poly-n-propyloxy, and amine, and Mono reduces adhesion interactions at near contact.
However, a typical solid lubricant, which has a high melting point and is considered thermally stable, disturbs an electromagnetic conversion process that is extremely highly sensitive, and moreover, an abrasion powder scraped by a head is generated on a running track. Therefore, abrasion properties are deteriorated. As described above, the liquid lubricant has mobility that enables to move the adjacent lubricant layer to replenish the lubricant removed due to abrasion by the head. However, the lubricant is span-off from a surface of the disk especially at a high temperature during driving of the disk, because of the mobility of the lubricant, and thus the lubricant is reduced. As a result, a protection function is lost. Accordingly, a lubricant having a high viscosity and low volatility is suitably used, and use of such a lubricant enables to prolong a service life of a disk drive with suppressing an evaporation rate.
Considering the above-described lubricating systems, requirements for a low-friction and low-abrasion lubricant used for thin film magnetic recording media are as follows.    (1) Low volatility.    (2) Low surface tension for a surface filling function.    (3) Interaction between terminal polar groups and a surface of a disk.    (4) High thermal and oxidization stability in order to avoid decomposition or reduction over a service period.    (5) Chemically inactive with metals, glass, and polymers, and no abrasion powder generated from a head or a guide.    (6) No toxicity and no flammability.    (7) Excellent boundary lubricating properties.    (8) Soluble with organic solvents.
Recently, an ionic liquid has been attracted attentions as one of solvents for synthesis of organic or inorganic materials and being friendly to the environments in the fields of electricity storage materials, a separation technology, and a catalyst technology. The ionic liquid is roughly classified as a molten salt having a low melting point. The ionic liquid is typically a molten salt having a melting point of 100° C. or lower, among the above-mentioned molten salts. The important properties of the ionic liquid used as a lubricant are low volatility, inflammability, thermal stability, and an excellent dissolving performance. Accordingly, because of the characteristics of the ionic liquid, the ionic liquid is expected to be applicable as a novel lubricant used in an extreme environment, such as in vacuum, and high temperature. Moreover, known is a technique where a controllability of a transistor is enhanced 100 times a controllability of a conventional transistor by using an ionic liquid in a gate of a single self-assembled quantum dot transistor. In this technique, the ionic liquid forms an electric double layer, which functions as an insulating film of about 1 nm, to thereby obtain a large capacitance.
Among others, there have been a considerable number of reports on an imidazole-based aprotic ionic liquid. For example, abrasion and wear of a surface of a metal or ceramic may be reduced by using a certain ionic liquid compared to a conventional hydrocarbon-based lubricant. For example, there is a report that, in the case where an imidazole cation-based ionic liquid is synthesized by substituting with a fluoroalkyl group, and tetrafluoroboric acid salt or hexafluorophosphoric acid salt of alkyl imidazolium is used for steel, aluminium, copper, single crystal SiO2, silicon, or sialon ceramics (Si—Al—O—N), tribological properties more excellent than those of cyclic phosphazene (X-1P) or PFPE are exhibited. Moreover, there is a report that an ammonium-based ionic liquid reduces frictions more than a base oil in the region of elastohydrodynamic to boundary lubrication.
Also, there has been proposed a synthetic lubricant obtained by adding an imidazole-based compound having a long-chain alkyl group to an aprotic ionic liquid containing bis(fluorosulfonyl)imideimidazolium as a main component (see PTL 1). In this proposed technique, observation of abrasion traces generated on a test steel plate after completion of an abrasion test confirms that the proposed synthetic lubricant provides a lowered coefficient of friction and improved abrasion compared to a system free of the imidazole-based compound having a long-chain alkyl group.
Also, most of the ionic liquid-based lubricants having imidazolium have been reported to be based on tetrafluoroborate [BF4]− which is an anion based on boron (see NPL 1 to NPL 5).
Also, an aprotic ionic liquid containing BF4− has been reported to have significantly favorable tribological performances in steel-steel contacts and steel-aluminum contacts (see NPL 6).
These reports suggest that BF4− has excellent tribological performances, but unfortunately its detailed mechanism is not described.
Also, there has been proposed a lubricant exhibiting excellent tribological performances at 20° C. and 100° C. in steel-steel contacts compared to conventional high-temperature lubricants such as X-IP and perfluoropolyether (PFPE) (see NPL 7).
However, BF4− is hydrophilic and has high sensitivity to moisture, and thus is not desired in tribology and other industrial applications. These kinds of anions are very sensitive to moisture and can be hydrolyzed to form hydrogen fluoride. These kinds of products cause erosion via various tribochemical reactions, which may cause damages on the substrates in machine systems. Therefore, it is necessary to increase hydrophobicity of anions to decrease their reactivity to moisture and provide a lubricant having tribological properties excellent even in various environments.
The following are proposed as imidazolium-based novel ionic liquids containing an anion fluoride, which are useful as ionic liquids.
Specifically, examples include an imidazole derivative ionic liquid having a fluorine-based end (see NPL 8) and an imidazole derivative ionic liquid having bis(trifluoromethanesulfonate)amide anion (see NPL 9).
There is also proposed a partially-fluorinated sulfonate-based aprotic ionic liquid having an octadecyl group at position 1 (see PTL 2). However, in this proposed technique, actual application properties are not described.
Moreover, effects of the ionic liquid as an additive for a base oil have been studied, and a chemical or tribochemical reaction of the ionic liquid has been researched to understand lubricating systems. However, there are almost no application examples of the ionic liquid to magnetic recording media.
Meanwhile, a protic ionic liquid (PIL) is a collective name of a compound formed by a chemical reaction between Bronsted acid and an equivalent amount of Bronsted base. It has been reported that perfluorooctanoic acid alkyl ammonium salt is PIL, and has a significant effect of reducing frictions of a magnetic recording medium compared with the above-mentioned Z-DOL (see PTL 3 and PTL 4, and NPL 10 to NPL 12).
Moreover, protonic ionic liquids are synthesized more easily than aprotic ionic liquids. For example, protonic ionic liquids have no need to synthesize a quaternary salt of nitrogen and can be synthesized simply by mixing equimolar amounts of an acid and a base. As a result, for example, possible molecular designs for, for example, increasing thermal stability become very variable.
Reported is a lubricant for a magnetic recording medium where thermal stability of the lubricant is enhanced by making a difference (ΔpKa) between pKa of acid and pKa of base large using sulfonic acid ammonium salt (see NPL 13). In this report, it has been confirmed that a mechanism of thermal stability of the lubricant is different depending on a value of ΔpKa, and a weight loss is endothermic and the weight loss occurs due to evaporation in the case where a value of ΔpKa as measured by DG/DTA is small, whereas a weight loss is exothermic and the weight loss is dominantly caused by thermal decomposition in the case where a value of ΔpKa is large.
Meanwhile, hard disks have been developed aiming the surface recording density of from 1 Tb/in2 to 2.5 Tb/in2. Currently, developments of techniques for large capacities of recording media have been actively performed with reduction in a size of magnetic particles as a premise. As a technique for a large capacity of a recording medium, there are techniques, such as reduction in an effective flying height, and introduction of Single Write (BMP).
As a recording technique of the next generation, moreover, there is “heat assisted magnetic recording.” FIG. 3 illustrates a schematic view of heat-assisted magnetic recording. In FIG. 3, reference numeral 1 is laser light, reference numeral 2 is near-field light, reference numeral 3 is a recording head (PMR element), and reference numeral 4 is a reproducing head (TMR element).
Examples of a problem of this technique include a deterioration of durability due to evaporation or decomposition of a lubricant present on a surface of a magnetic layer, because a recording area is heated with laser at the time of recording and reproducing. In heat-assisted magnetic recording, a recording medium may be exposed to a high temperature, such as 400° C. or higher, even though it is for a short period. Therefore, thermal stability of a lubricant is concerned, if the lubricant is a typically used lubricant Z-DOL for thin film magnetic recording media, or a carboxylic acid ammonium salt-based lubricant.